Detection of QPO in the peak of outburst of black hole candidate Swift J1727.8-1613: AstroSat/LAXPC Observation in Slew Mode

ATel #16235; Tilak Katoch (TIFR, Mumbai), H. M. Antia (CEBS/TIFR, Mumbai), Anuj Nandi (U R Rao Satellite Centre, Bengaluru), Parag Shah (TIFR, Mumbai)
on 10 Sep 2023; 17:26 UT
Credential Certification: Tilak Katoch (tilak@tifr.res.in)

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Swift J1727.8-1613 X-ray transient source was detected on 24 August 2023 as reported in GCN #34540 and ATel (#16205, #16206, #16231). It was classified as a low mass black hole X-ray binary with subsequent observation in optical (ATel #16208), X-ray (ATel #16207), and radio (ATel #16211).

The source is currently too bright (> 6 Crab) to be observed directly by AstroSat/LAXPC, but AstroSat/LAXPC instrument observed this source during a slew operation with an offset of about 0.9 degree on 2^nd September 2023. The spectrum obtained was modelled with a power-law component to characterize the emission from the compact corona along with absorbed Galactic neutral gas abundance as a free parameter for the energy range 3-30 keV. Preliminary spectral modeling with the absorbed power-law (Tbabs*powerlaw) indicates statistically good fit with χ²/dof= 57.48/39. The model fitted spectrum with residual is shown in Figure 1 (https://www.tifr.res.in/~astrosat_laxpc/5829-lxp-spec.jpg).

The 3-30 keV observed flux during the observation was 2.7x10^-7 ergs/s/cm² along with photon index ~ 2.3, indicating the softer spectral nature with power-law dominated during the peak of the outburst. The measured column density corresponds to an unabsorbed flux of 3.0x10^-7 ergs/s/cm².

Preliminary timing analysis shows a prominent signature of QPO feature in the power density spectrum. Modeling of power spectrum reveals QPO feature at 1.1 Hz (Q ~ 12.8, Sigma ~ 4.8) along with harmonic at 2.0 Hz (Q ~ 9.9, Sigma ~ 2.4). The model fitted power spectrum is shown in Figure 2 (https://www.tifr.res.in/~astrosat_laxpc/5829-lxp-pds.jpg).

LAXPC was built by TIFR in collaboration with the Indian Space Research Organisation (ISRO). The Indian Space Research Organisation funded, managed and facilitated the project.